Pressure-sensitive copying paper, also called noncarbon paper, comprises an upper sheet coated with a dispersion of microcapsules containing an electron donating colorless dye solution and a lower sheet coated with a dispersion of an electron accepting color developer and, if desired, an intermediate sheet coated on each side thereof the microcapsule dispersion and the color developer dispersion, respectively. On application of pressure, the microcapsules are ruptured, and the colorless dye reacts with the color developer to form a color image.
Known color developers which have been put into practical use include inorganic solid acids, e.g., active clay, acid clay, and attapulgite (see U.S. Pat. No. 2,712,507), p-substituted phenolic resins, e.g., p-phenylphenol-formaldehyde condensates (see JP-B-42-30144, the term "JP-B" as used herein means an "examined Japanese patent publication"), aromatic carboxylic acid metal salts, e.g., zinc 3,5-di-.alpha.-methylbenzylsalicylate (see JP-B-49-10856 and JP-B-52-1327), and metal salts of phenolic resins, e.g., a zinc-modified p-phenylphenol-formaldehyde condensate (see JP-B-55-25998). However, the inorganic solid acids, though rapidly acting in color formation, adsorb gases or water content in air during preservation to reduce the color developability. The p-substituted phenolic resins, e.g., p-phenylphenol-formaldehyde condensates, though excellent in color developability, undergo yellowing on exposure to light or gases in air. The zinc modified p-phenylphenol-formaldehyde resins suffer from more serious yellowing. Further, although the aromatic carboxylic acid metal salts exhibit excellent color developability and freedom from yellowing, they have disadvantages of poor developed image water-resistance and expensiveness.